Resin transfer molding has been around for many decades, and its use has grown considerably in recent years. The process allows the economical manufacture of high quality composites. In accordance with the process, a resin system is transferred at low viscosities and low pressures into a closed mold die containing a preform of dry fibers. The dry fibers, which may have the form of continuous strand mat, unidirectional, woven, or knitted preforms, are placed in a closed mold and resin is introduced into the mold under external pressure or vacuum. The resin cures under the action of its own exotherm, or heat can be applied to the mold to complete the curing process.
The resin transfer molding process can be used to produce low-cost composite parts that are complex in shape. These parts typically provide continuous fiber reinforcement, along with inside mold line and outside mold line controlled surfaces. It is the placement of the continuous fiber reinforcements in large structures that sets resin transfer molding apart from other liquid molding processes.
In the past, resin transfer molding was used for applications suitable to consumer product markets. However, in the last few years, through the development of high-strength resin systems and more advanced pumping systems, resin transfer molding has advanced to new levels. These recent developments have promoted resin transfer molding technology as a practical manufacturing option for high-strength composite designs, particularly in the aerospace industry.
In the aerospace industry, the most visible advantage to the resin transfer molding process lies in resin transfer molding's ability to combine multiple, detailed components into one configuration. For example, many traditional designs consist of many individual details that are combined as a subassembly. These subassemblies usually require labor-intensive shimming, bonding, mechanical fastening, and sealing. Consequently, these subassemblies demonstrate high part-to-part variability due to tolerance build-up.
Resin transfer molding produces an aerodynamic, decorative finish, with controlled fit-up surfaces. Being a product of the mold makes the surface quality of the part produced within the mold comparable to that of the tool's surface.
Resin transfer molding also provides control of the fiber/resin ratio in the completed product. This advantage produces parts that are lightweight and high in strength.
Unlike conventional composite systems that use lay-up of prepreg materials, resin transfer molding does not require an autoclave. Therefore, no autoclave costs are incurred, no size limitations are inherent, and no staging issues occur.
In terms of raw material cost, resin transfer molding offers cost savings by using bulk materials like broad goods. Because dry goods are less expensive than preimpregnated materials, savings can be associated with the cost of the wasted material during the ply-knitting operation. Also, bulk materials do not require special handling requirements such as freezer storage.
The basic injection operation of resin transfer molding is straightforward and easily learned. Hence, minimal training is required to bring operators on line. On the other hand, in making preforms, the level of operator skill and training is dependent upon the method of preforming that is used. Preform fabrication methods include braiding, knitting, weaving, filament winding, and stitching. Each of these methods is quite different and must be individually evaluated for specific design characteristics.
The initial capital investment costs of resin transfer molding are low when compared with many other molding processes. An elementary form of resin transfer molding can be achieved using a pressure pot, an oven, and a vacuum source. A variety of commercially available equipment can be used to enhance the process in many areas.
In most cases, resin transfer molded materials can be formed with minimal chemical exposure to workers and their environment. Many high-performance resin systems are stable and release low volatiles. Since resin transfer molding is processed within a closed system, workers are exposed to the resin only when loading the dispensing equipment.
One of the problems encountered when using resin transfer molding is that complex cavities that extend into the surface of the part must be formed in the mold cavity surface, or the complex cavity will be filled by resin during the resin injection process. If the complex cavity is designed to receive a bushing or an insert, the bushing or insert can be incorporated into the preform and injected in place to eliminate some higher level assembly and to avoid the need for a complex tooling surface. If the part includes an internal hollow tube, proper design of the tool to take this into account may be difficult and expensive, or may produce a tooling configuration from which removal of the finished part would be difficult.
Other problems are encountered in laying up or arranging preforms of fibers prior to placing the preform into the mold. If braided or woven fabric is used, cutting of that fabric often results in frayed edges, which is undesirable. Arranging stacks, or tapered-off sections of the preforms on a substrate so that ply drops are aligned correctly is also difficult.
The present invention solves many of the above problems by providing a series of unique processes for the fabrication of a wind tunnel blade. The processes result in a new structure for a wind tunnel blade.
It has become conventional practice in the aircraft industry to manufacture helicopter and other blades having a molded fiber-reinforced resin body formed by resin transfer molding. The fiber-reinforced resin bodies were often formed about an internal, metallic, load-bearing spar. Such fiber-reinforced resin bodies exhibited high strength and low weight characteristics. With the exception of the internal metal spar, however, prior art resin transfer molded rotor blades did not include structural reinforcements along their length.
Prior art wind tunnel blades were formed from a lay-up of prepreg composite material that was shaped into a unitary structure including a base attached to the blade. The housing and the hub for the wind tunnel blades required that a technician lay on his back and install the unitary base and blade structure into the wind tunnel's hub, which was difficult.
Because prior art wind tunnel blades were subjected to high speed wind conditions, the wind tunnel blades were often damaged as a result of fatigue and wind erosion. To counter this wind erosion, the prior art wind tunnel blades included frangible foam tips at their distal ends. The frangible foam tips were often formed of a foam material having a uniform density. The frangible foam tip was wrapped in plies of fiberglass to protect the foam from wind erosion and to improve impact resistance. This wrapped fiber piece was difficult to form, and required a large amount of labor to produce.
Prior art wind tunnel blades were difficult to balance because the wind tunnel blades were not of uniform weight and did not have consistent centers of gravity. The prior art wind tunnel blades were balanced by adding lead weights to the blade butt to adjust the center of gravity. After the center of gravity was adjusted, the blade must be matched to another blade of approximately the same weight. This matching process can be difficult because of the large blade-to-blade variation in weight.
The present invention solves the above problems by providing a novel wind tunnel blade design incorporating a variety of different features that permit easier installation, service, and replacement of the wind tunnel blades. The process of forming the unique wind tunnel blade incorporates a number of new composites forming techniques. These techniques are applicable to a number of parts or products, and can be used to form parts having a number of different configurations or complex shapes.